Scelidosaurus (; with the intended meaning of "limb lizard", from Greek skelis/σκελίς meaning 'rib of beef' and sauros/σαυρος meaning 'lizard') is a genus of herbivorous armoured ornithischian dinosaur from the Jurassic of England. Scelidosaurus lived during the Early Jurassic epoch, during the Sinemurian to Pliensbachian ages around 191 million years ago, at the time when Europe formed an island archipelago. Its fossils have been found in the Charmouth Mudstone Formation near Charmouth in Dorset, England, and these fossils are known for their excellent preservation. Scelidosaurus has been claimed as one of the earliest complete dinosaurs, and is among the most completely known dinosaur of the British Isles. Despite this, a modern description only materialised in 2020. After initial finds in the 1850s, comparative anatomist Richard Owen named and described Scelidosaurus in 1859. Only one species, Scelidosaurus harrisonii named by Owen in 1861, is considered valid today, although one other species was proposed in 1996. Scelidosaurus was about 4 metres (13 ft) long. It was a largely quadrupedal animal, feeding on low scrubby plants, the parts of which were bitten off by the small, elongated head to be processed in the large gut. Scelidosaurus was lightly armoured, protected by long horizontal rows of keeled oval scutes that stretched along the neck, back and tail. One of the oldest known and most "primitive" of the thyreophorans, the exact placement of Scelidosaurus within this group has been the subject of debate for nearly 150 years. This was not helped by the limited additional knowledge about the early evolution of armoured dinosaurs. Today most evidence suggests that Scelidosaurus is the most derived of the known basal thyreophorans, either closely related to Ankylosauria or Stegosauria+Ankylosauria.
Description
Size and posture
A full-grown Scelidosaurus was rather small compared to most later non-avian dinosaurs, but it was a medium-sized species in the Early Jurassic. Some scientists have estimated a length of 4 metres (13 ft). In 2010, Gregory S. Paul gave a body length of 3.8 metres (12.5 ft) and a weight of 270 kg (600 lb). Scelidosaurus was quadrupedal, with the hindlimbs longer than the forelimbs. It may have reared up on its hind legs to browse on foliage from trees, but its arms were relatively long, indicating a mostly quadrupedal posture. A trackway from the Holy Cross Mountains of Poland shows a scelidosaur like animal walking in a bipedal manner, hinting that Scelidosaurus may have been more proficient at bipedalism than previously thought.
Distinguishing traits The first modern diagnosis was provided by David Bruce Norman in 2020. In a first article, Norman provided autapomorphies, unique derived characters, of the skull. The front snout bones, the premaxillae, have a common central rough extension, in life bearing a small upper beak. The nasal bone has on its upper outside a facet touching the inner side of the ascending branch of the premaxilla. The antorbital fenestra is present as a bean-shaped depression, its lower edge formed by a sharp ridge. The central parietal crest on the skull roof is formed by two parallel crests separated by a narrow trough on the midline. The roof of the nasal cavity is formed by special plates above the vomers, called the "epivomers". The epipterygoid bone is shaped as a small conical vertical structure of which the base connects to the upper side of the pterygoid bone by means of a lateral flat surface. The basioccipital has large oblique facets on the lower sides. The opisthotic has an expanded pedicel with facets on its underside. Elongated epistyloid bones project obliquely to the rear and below, from the back of the skull. A small spur-like structure on the upper edge of the paroccipital process encases the posttemporal fenestra. The rear of the skull is fused on its upper edge with a pair of large curved horn-shaped osteoderms. The lower jaw shows only little exostosis, limited to the angular, and lacking an attached osteoderm.
Skull
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